The present invention relates to organic light-emitting diode devices and more particularly to improving the luminance efficiency of such a device.
Organic light-emitting diodes (OLEDs) are a class of electronic devices that emit light in response to electrical current applied to the devices. The structure of an OLED generally comprises, in sequence, a glass support, a transparent anode, an organic hole-transporting layer (HTL), an organic light-emitting layer (EML), an organic electron-transporting layer (ETL), and a metal cathode. Tang et al, in Applied Physics Letters, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly assigned U.S. Pat. No. 4,769,292, demonstrated highly efficient OLED using such a layer structure. In order to operate the device with a higher efficiency, a balanced carrier recombination in the light-emitting layer is needed in the device. By balanced carrier recombination, it is meant that the rate of arrival of the number of holes in the emitting layer is preferably equal to the rate of arrival of the number of electrons. In this manner, the probability of holes meeting with electrons in the light-emitting layer is optimized. Recombination of these holes and electrons produces light, also known as electroluminescence. However, OLED devices with multilayer structure as described by Tang et al. in the above references do not necessarily provide the configuration to achieve a balanced carrier recombination. One of the reasons is that the hole carriers have a tendency to be more mobile than the electron carriers, therefore, the number of holes injected into the light-emitting layer is considerably larger than the number of electrons injected into this layer. As a result, the electron-hole recombination events are not optimized and the efficiency of electroluminescence is reduced. Furthermore, it has been shown by Aziz et al. (Science, 283, 1900 [1999]) that injection of excessive holes into the light-emitting layer is undesirable as it could lead to the degradation of the OLED device. Also, a direct contact between the hole-transporting layer and the light-emitting layer could lead to a non-uniform intermixing of materials between these layers, resulting in the formation of non-emissive sites known as dark spots. This observation was reported by Fujihira et al. (Applied Physics Letters, 68, 1787 [1996]).
Various efforts have been made to improve cathode and to improve electron-transporting materials in order to get more efficient light-emission from the OLEDs. For example, Tang et al. (U.S. Pat. No. 4,885,211) and VanSlyke et al. (U.S. Pat. No. 5,059,862) disclosed several kinds of cathodes with reduced electron injection barriers. Shi et al. (U.S. Pat. No. 5,766,779) disclosed new electron-transporting organic materials with higher electron mobility. However, under the normal operational conditions, the electron-mobility in these improved electron-transporting materials are still relatively low in comparison with the hole-mobility of the common hole-transporting materials used in the OLED device. Thus, even if both the anode and the cathode were perfectly injecting charged carriers, meaning that both contacts are ohmic, the large difference in the hole-mobility and the electron-mobility will result in an unbalanced injection of the number of holes and electrons into the light-emitting layer. Consequently, the electroluminescence efficiency is poor.
It is an object of the present invention to provide an OLED with improved efficiency.
This object is achieved in an organic light-emitting device comprising, in sequence:
a) a substrate;
b) an anode;
c) a hole-transporting layer having a hole-transporting organic compound;
d) an interface layer in a range of thickness between 0.1 nm to 5 nm;
e) a light-emitting layer having a light-emitting organic compound;
f) an electron-transporting layer having an electron-transporting organic compound; and
g) a cathode;
wherein the interface layer contains a compound having an ionization potential greater than that of the organic compound of the hole-transporting layer, and an energy bandgap equal to or greater than that of the organic compound of the light-emitting layer.
An advantage of the present invention is that OLED devices with improved luminance efficiency are produced. It has been found quite unexpectedly that the interface layer disposed between the hole-transporting layer and the light-emitting layer in the range of thickness between 0.1 nm and 5 nm can significantly improve the luminance efficiency of the OLED device. This interface layer provides a mechanism wherein the injection of holes and electrons into the light-emitting layer is optimized and therefore a balanced recombination of electrons and holes is achieved.